1. Field of the Invention
The present invention relates generally to parylene polymer layers, and more specifically to such layers which are disposed between electrically conductive layers in integrated circuits.
2. Discussion of the Related Art
Semiconductors are widely used in integrated circuits for electronic products systems such as computers and televisions. These integrated circuits typically combine many transistors on a single crystal silicon chip to perform complex functions and store data. Semiconductor and electronics manufacturers, as well as end users, desire integrated circuits which can accomplish more in less time in a smaller package while consuming less power. However, some of these desires may be in opposition to each other. For example, simply shrinking the feature sizes on a given circuit from 0.5 microns to 0.25 microns can increase power consumption by 30%. Likewise, doubling operational speed generally doubles power consumption. Miniaturization also generally results in increased capacitive coupling, or cross talk, between conductors which carry signals across the chip. This can limit achievable speed and degrade the noise margin used to insure proper device operation.
One way to diminish power consumption and cross talk effects is to decrease the dielectric constant of electrically insulating layers that separate layers of electrically conductive materials within the integrated circuit. In addition, since operational conditions may include high temperatures, it can be advantageous to use materials with relatively high thermal stability to form the insulating layers.
Silicon dioxide is one of the most common materials used in insulating layers for integrated circuits. However, silicon dioxide is non-ideal due to its comparatively high dielectric constant of about 3.9.
Layers formed from parylene polymers have been proposed for use as insulating layers in integrated circuits because these materials have relatively low dielectric constants and comparatively high melting temperatures. Parylene polymers are poly-p-xylylenes which may be prepared starting with a dimer having the structure: ##STR2## wherein X is typically a hydrogen or a halogen. The most common forms of parylene dimers include the following: ##STR3##
Polymer films formed from these parylene dimers may have comparatively low dielectric constants and relatively high melting temperatures. However, these parylene polymer films have relatively high dielectric constants and inferior thermal stabilities, rendering them less attractive for use in integrated circuits.
Typically, a vapor deposition method is used to form parylene polymer layers from parylene dimers. One such vapor deposition method is disclosed in Journal of Applied Polymer Science 13, 2325 (1969). According to this method, commonly referred to as the Gorham process, the parylene dimer is cracked at an elevated temperature to produce parylene monomer having the structure: ##STR4## The parylene monomer is condensed onto a substrate at a temperature of from about room temperature to about -35.degree. C. Under these conditions, the parylene monomer simultaneously polymerizes on the substrate to form a layer of the parylene polymer adhered to the substrate.
A vapor deposition process for forming poly-p-xylylene films from other than parylene dimer materials is disclosed in U.S. Pat. No. 5,268,202 (You). The method disclosed by this reference includes forming the monomer vapor inside the vacuum chamber in which deposition occurs, precluding the purification opportunities afforded by the parylene dimer process prior to deposition on the substrate. Therefore, the resulting poly-p-xylylene layers have relatively high impurity levels. For example, Journal of Vacuum Science and Technology Al1(6), 3057 (1993) discloses that poly-p-xylylene films formed by the method of You have zinc impurity levels of about 3%. Such a comparatively high level of zinc impurity results in problems with the poly-p-xylylene film relating to, for example, increased dielectric constant, decreased surface resistivity, ion movement within the layer, film surface charging effects and/or electron storm formation when the poly-p-xylylene film is used within a multi-level structure. As a result, these poly-p-xylylene films cannot be used in integrated circuits.
Hence, it remains a challenge in the art to provide an electrically insulating layer for use in integrated circuits of electronic devices that has a relatively low dielectric constant and comparatively high melting temperature. It is a further challenge in the art to provide such an insulating layer which is formed from a parylene polymer.